The present invention concerns a shadow mask, and more particularly a pattern of electron beam transmission holes of the shadow mask.
Generally, a three-gun color picture tube comprises, as shown in FIG. 1, a three-gun assembly 8 of three electron guns 9 arranged linearly or in a triangular form, a deflecting system 6 for deflecting the electron beams emitted from the three-gun assembly, a shadow mask 3 having a number of electron beam transmission holes 5, and a panel 1. The electron beams 7 passing the holes 5 strike the color phosphors of red, green and blue deposited on the inner surface 2 (screen) of the panel 1.
The shape of the phosphor corresponds to that of the electron beam transmission hole 5, and the mutual positions of the three color phosphors 4, struck by three electron beams 7 passing one hole 5, correspond to the arrangement of the three electron guns 9.
The arrangement of the color phosphors 4 on the screen 2 is determined by the arrangement of the electron beam transmission holes 5 in the shadow mask 3.
Usually, the electron beam transmission holes 5 have a circular or a rectangular shape. The rectangular holes are generally arranged as shown in FIG. 2. The holes 5 have the vertical pitch Py, separated from each other by width b of bridge 10. The pitch of two adjacent rows of the holes are vertically offset from each other by the amount of .DELTA.y.
The transmissivity of the electron beams passing through the holes is maximum when the scanning lines 11 pass the centers of the holes, and minimum when the centerline between two adjacent scanning lines 11 corresponds to the center of the holes 5, respectively as shown in FIGS. 3A and 3B. This is represented by the following formula: ##EQU2## where Ps represents the pitch of the scanning lines, Py the vertical pitch of the electron beam transmission holes, and n integer.
According to the above formula, the contrast variation having the vertical scanning period of 2n-1, i.e., moire period is determined by the ratio of the vertical pitch Py to the interval h between the scanning lines 11. Also, the relative size of the moire is determined by the ratio of the width of the holes to the width of the scanning lines.
Thus, the interference between the vertical arrangement of the holes in the shadow mask and the scanning lines causes undulated patterns, i.e. moire effect to appear on the screen 2.
In order to reduce the moire effect, there have been many researches, one of which is dislcosed in the U.S. Pat. No. 4,210,842. According to this U.S. Patent, the vertical pitch of the holes is determined so as to reduce the moire pitch according to the broadcasting method, and the mask pattern is determined to obtain the moire phase difference. Then, the vertical pitches of the holes are arbitrarily arranged in the shadow mask so as to scatter the moire pattern. However, this technique greatly enhances the moire effect, and makes the process of producing the shadow mask difficult.
Moreover, the technique for designing a shadow mask specific for NTSC, PAL and SESAM according to the broadcasting methods slightly reduces the moire effect, but cannot resolve the deviations resulting from the overscanning of a set and the size of the electron beams, thereby not considerably reducing the moire effect.